This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This description is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of hydrocarbons from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants such as hydrogen sulfide (H2S) and carbon dioxide (CO2). When either H2S or CO2 is produced as part of a hydrocarbon gas stream, such as methane or ethane, the raw natural gas is sometimes referred to as a “sour” natural gas. The H2S and CO2 are often referred to together as “acid gases.”
The sour natural gas may be treated to remove the H2S, CO2, and other contaminants before it can be used as an environmentally-acceptable fuel. For “highly sour” natural gases (e.g., >20% CO2+H2S), it can be particularly challenging to design, construct, and operate a process that can economically separate these contaminants from the desired hydrocarbons. In many cases, the acid gas, which primarily contains H2S and CO2, may be sent to a sulfur recovery unit (SRU) to convert the toxic H2S into benign elemental sulfur, while the CO2 is usually just vented to the atmosphere. In certain instances, the CO2 could be captured, stored, and later sold for commercial use as a measure to prevent the introduction of nitrogen into the stream.
The Claus process is the most widely used process for the conversion of H2S to elemental S8, which can later be sold as a separate product from the natural gas. The acid gas that is fed to the Claus process is converted in two stages, a thermal stage, and a catalytic stage. The process can include multistage catalytic oxidation of H2S resulting in the following net overall reaction:3H2S+1.5O2→3H2O3S  (1)
To provide the necessary amount of oxidative component, the first step in the Claus process is the combustion of one-third of the H2S in the acid gas to form sulfur dioxide (SO2) in the thermal stage:H2S+1.5O2→SO2+H2O  (2)
FIG. 1 is a block diagram of the conventional Claus process where in the thermal stage, a low-pressure acid gas 102 may flow into a reaction furnace 104. In the reaction furnace 104, sufficient combustion air 106 may be introduced to burn about one-third of the H2S. The hot, partially-combusted acid gas, which may contain H2S, SO2, and gaseous sulfur (S), leaving the reaction furnace 104 may be used to produce steam in a waste heat boiler 108 resulting in the cooling of the acid gas. As shown in FIG. 1, reaction furnace 104 and waste heat boiler 108 may be shown as a single component since both may be housed in a single unit and heated by the same heater. The partially-combusted acid gas may be further cooled and condensed in a condenser 110 to form an unreacted acid gas stream 112. This condenser 110 also condenses gaseous Sx generated during the conversion of H2S to form liquid Sx 114, where x is typically an integer from 2 to 8. The liquid Sx 114 may be separated from the overhead unreacted acid gas stream 112, and removed from an outlet on the condenser 110. In this thermal stage, the pressure and temperature of the reaction furnace 104 may be maintained at about 5 to 15 psig (0.3-1 bar) and about 1800° F. (1,000° C.), respectively. At those conditions, the Claus reaction occurs thermally in the reaction furnace 104 without requiring any catalyst. Under such conditions, about 70% of the H2S in the acid gas 102 may be thermally converted into elemental Sx 114 within the reaction furnace 104.
The unreacted acid gas stream 112 enters the catalytic stage at a relatively low pressure. A portion of the remaining unreacted two-thirds H2S in the acid gas may be converted in the catalytic stage to form elemental S2 as follows:2H2S+SO2→2H2O+3S  (3)Summing reaction (2) with reaction (3) yields reaction (1).
As shown in FIG. 1, the low-pressure unreacted acid gas stream 112 from the condenser 110 may be directed to and processed in a reheater 116, a catalytic reactor 118, and a condenser 120 within the catalytic section configured to separate Sx 119. The sequence of gas reheat 122, catalytic reaction 124, condensation 126, and separation of liquid Sx 128 may be repeated in numerous catalytic stages at successively lower reactor temperatures. The remaining gas separated from the last condenser 126 is referred to as a tail gas 130 and may be burned in an incinerator or further desulfurized in a tail gas treatment unit (TGTU). For a well-designed Claus sulfur recovery plant having two or more catalytic reactors, an overall conversion of at least 95% of H2S to Sx can be achieved.
Alberta Sulfur Research, Ltd (ASRL) has recently described a process in which H2S is converted to elemental Sx at elevated pressures in a molten sulfur medium. However, the ASRL process is only intended for small tonnages of sulfur (<10 tons per day) and is not amenable to large acid gas streams.
Another process technique using high pressure acid gas pertains to larger amounts of H2S while maintaining CO2 at a high pressure for facile injection. This process involves the use of an ancillary solvent, which adds complexity to the process. Accordingly, there is a need for a process that is capable of processing a high-pressure acid gas under elevated heat of reaction conditions and recovering CO2 for viable downhole injection for a disposal process or for an enhanced oil recovery (EOR) process.